The Penetration of Light Into Wood

A characteristic of many woods exposed to strong sunlight is to change to a dark brown color on the surface. Four methods were used to measure transmission of light through wood. The first was to expose photosensitive film under varying thicknesses of wood. The second measured the light intensity transmitted through wood with a photovolatic cell. The third and fourth were spectrophotometric methods in which the spectrum of transmitted light was measured after passing through wood. The woods studied were Larix occidentalis Nutt., Sequoia sempervirens (D. Don Endl.) and Pinus ponderosa Laws. All methods gave results that were in agreement except that the photographic method yielded extinction coefficients for visible and infrared wavelengths. Penetration of ultraviolet light into wood is only a few mils, and less than the thinnest veneers that can be cut. Transmission of visible light varies widely for different woods and parallels the depth of color. Infrared light 750 to 2,000 millimicrons penetrates wood more deeply than visible light. For both kinds of light the variation in transmission among the woods tested is small. Visible light is transmitted somewhat more readily through sapwood than heartwood of ponderosa pine. Visible light browns wood to a greater depth than UV light. As the brown color develops it acts as a barrier to visible light and transmission below 500 millimicrons wavelengths practically ceases. Only about l.6 x 10-2 percent of visible light reaches a depth of 0.l inch in Larix occidentalis Nutt. and 6 x 10-8 percent reaches that depth in Sequoia sempervirens. If visible and infrared light are taken together, 0.28 percent reaches 0.1 inch in the former and 0.04 inch in the latter species.

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